Approximately 20-50% of older adults (age > 65 years) are living with a rotator cuff tear, and this number increases with advanced age. A rotator cuff tear is associated with decreased muscle volume and shoulder strength, fatty infiltration, reduced range of motion, and limited mobility. These deficits can impact older adults ability to perform activities of daily living (ADLs), resulting in a loss of independence. It has been suggested that individuals with a torn rotator cuff will employ compensatory movements or deviations from the desired movement with a greater reliance on unimpaired muscles, to complete tasks. However, the aging process induces changes to skeletal muscle which may affect motor coordination and muscle strength, contributing to compensation seen in healthy aging. Therefore, it is essential to identify the extent to which upper limb compensation is necessary with age to further explore the functional changes occurring in older adults with a rotator cuff tear. Clinically, standardized functional outcome scores are used to assess functional ability and improvements following treatment for patients with a rotator cuff tear. However, it is not well understood how these clinical metrics relate to compensation in ADL tasks. To better understand the relationships between healthy aging, rotator cuff tears, and functional ability, it is necessary investigate compensation in ADLs and identify the strength and activation of primary muscles contributing to these movements. These objectives will be accomplished by obtaining kinematics for 6 ADL tasks from healthy and rotator cuff injured older adults and developing subject-specific musculoskeletal models for these subjects. These models will be used to simulate the ADL movements to identify the forces and activation patterns of muscles contributing to these tasks. Specifically, this study will 1) evaluate the effet of rotator cuff tear on muscle volume, fatty infiltration and functional outcome scores, 2) identif differences in joint kinematics for 6 ADLs and evaluate the range of motion, and force and activation of contributing muscles, and 3) evaluate the dependence of range of motion and muscle force on muscle volume, fatty infiltration, and functional outcome score. It is of interest o study compensation in older adults with a rotator cuff tear to further understand the functional consequences of this injury. The long term goal of this study is to identify weak muscles that contribute to compensation or the inability to complete a functional task. This information will be used to design rehabilitation strategies to strengthen muscles and improve function after injury. The outcomes of this work will provide further understanding of the mechanism of injury in older adults with a rotator cuff tear. This information may have a substantial impact as the US population continues to age. PUBLIC HEALTH RELEVANCE: Many older adults (age > 65 years) experience a rotator cuff tear, which can affect their performance of daily tasks, like reaching to a shelf or combing their hair. The objective of this study is to better understand the factors contributing to the ability o older adults with a rotator cuff tear to perform daily tasks. As the US population continues to age, the information gained from this study will provide information about the ability of older adults with a rotator cuff tear to function independently.